In a standard ring-spinning machine respective filaments are wound up onto respective tubes on respective spindles carried in a long row on a bank of such spindles, and in some arrangements filaments are pulled off rows of tubes similarly mounted on spindles. An equally long doffing apparatus has respective grabs that engage the tops of the tubes and remove same from the spindles, while a similar such machine or even the same one can mount replacement tubes on the spindles.
It is clearly critical that the machine not try to load a new tube on a spindle where for some reason there is still a tube that was not removed in an earlier doffing cycle. Accordingly devices such as described in U.S. Pat. No. 3,791,124 of S. Kaufmann have a tube-detector comprised of a light beam that is directed the length of the spindle bank just above the upper ends of the spindles. Any tube, whether empty or full, left on a spindle will interrupt the beam and thereby indicate to a connected control system that something is wrong, so that donning and/or doffing can be interrupted while the problem is attended to, normally manually.
This system has several drawbacks. First of all it does not indicate which or how many spinning spindles are still carrying tubes. Thus the alarm and shutdown is triggered if there is a massive failure leaving all spindles full, or if only one spindle is left carrying a tube.
Another considerable disadvantage is that it does not indicate that a given spindle never had a tube, so that the machine can continue and will wind on filament onto the naked spindle. When a subsequent effort is made to fit a tube to this spindle, same will be blocked by the yarn and will break the new tube and/or the apparatus, creating a considerable problem.